1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive that characterizes misaligned servo wedges.
2. Description of the Prior Art and Related Information
Today, computing devices such as personal computers, personal digital assistants, cell-phones, etc., are routinely used at work, at home, and everywhere in-between. Computing devices advantageously enable the use of application specific software, file sharing, the creation of electronic documents, and electronic communication and commerce through the Internet and other computer networks. Typically, each computing device has a storage peripheral such as a disk drive.
A huge market exists for disk drives for mass-market computing devices such as desktop computers and laptop computers, as well as small form factor (SFF) disk drives for use in mobile computing devices (e.g. personal digital assistants (PDAs), cell-phones, digital cameras, etc.). To be competitive, a disk drive should be relatively inexpensive and provide substantial capacity, rapid access to data, and reliable performance.
Disk drives typically employ a moveable head actuator to frequently access large amounts of data stored on a disk. One example of a disk drive is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable transducer head for reading and writing data. The HSA forms part of a servo control system that positions the moveable transducer head over a particular track on the disk to read or write information from and to that track, respectively.
Typically, a conventional hard disk drive includes a disk having a plurality of concentric tracks. Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information may be provided on each disk to determine the position of the moveable transducer head.
The most popular form of servo is called “embedded servo” wherein the servo information is written in a plurality of servo wedges that are angularly spaced from one another and are interspersed between data sectors around each track of each disk.
Each servo wedge typically includes a phase lock loop (PLL) field, a servo synch mark (SSM) field, a track identification (TKID), a wedge ID field having a binary encoded wedge ID number to identify the wedge, and a group of servo bursts (e.g. an alternating pattern of magnetic transitions) which the servo control system of the disk drive samples to align the moveable transducer head with or relative to a particular track. Typically, the servo control system moves the transducer head toward a desired track during a course “seek” mode using the TKID field as a control input.
Once the moveable transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the moveable transducer head over that track in a fine “track follow” mode. During track following mode, the moveable transducer head repeatedly reads the wedge ID field of each successive servo wedge to obtain the binary encoded wedge ID number that identifies each wedge of the track. In this way, the servo control system continuously knows where the moveable transducer head is relative to the disk.
In contemporary hard disk drives employing embedded servos, it is well known to provide framing of servo data via the servo synch mark (SSM) field. For example, in hard disk drives, a servo synchronization signal based on the moveable transducer head reading a servo synchronization mark (SSM) results in a read/write channel of the disk drive establishing a precise timing reference point for the reading of servo data and for read/write operations.
Today, disks, especially for small form factor (SFF) disk drives, are increasingly being servo-written by external media servo writers before being assembled into disk drives. During external media servo-writing, multiple disks are simultaneously servo-written to without having to be located in a disk drive. Typically, this occurs in a clean-room environment.
The external media servo writer typically controls a rotatable actuator assembly including actuator arms having one or more heads respectively attached to each actuator arm, in which the actuator assembly rotates about a pivot such that the heads are radially positioned over the disks, respectively, in order to write servo wedges onto the disk based upon a timing clock. Thus, servo wedges are written to disks based upon actuator arms and heads rotating about a pre-defined pivot.
Unfortunately, when the externally servo-written disks are later assembled into a disk drive, servo wedge misalignment often occurs due to the different mechanical characteristics of the actuator assemblies of the external servo writer and the disk drive. Particularly, the heads of the disk drive rotate about a different pivot than the heads that were used in the prior external media servo writing process.
Because of this, timing uncertainties are introduced into the servo control system thereby causing problems in seek operations. More particularly, due to this servo wedge misalignment, the servo control system may often be very inefficient in searching for and locking onto servo wedges resulting in long time delays, or in a worse case may never lock onto a servo wedge, resulting in the failure of the disk drive.